1. Field of the Invention
The present invention relates to a hydrodynamic bearing device, a spindle motor and a hard disk driving apparatus having the hydrodynamic bearing device.
2. Description of the Prior Art
In recent years, recording apparatuses that utilize a rotating record medium such as an optical disk or a magnetic disk have been developed with larger memory capacity and higher data transfer rate. For this reason, disk driving apparatuses are required to rotate at a high speed with low NRRO (nonsynchronous run-out) and little rotation noise. Therefore, a hydrodynamic bearing device is used for the bearing.
In the hydrodynamic bearing device, oil, as a lubricant, exists between a shaft and a sleeve. A pumping pressure generated by a dynamic pressure generating groove due to rotation keeps the shaft in the state of noncontact with the sleeve. Since the shaft does not contact the sleeve in the hydrodynamic bearing device, the shaft has a high durability and is suitable for high-speed rotation due to a very small frictional resistance.
With reference to FIG. 9, a typical example of the conventional hydrodynamic bearing device will be described below. FIG. 9 is a cross sectional view of a hard disk driving apparatus including a conventional hydrodynamic bearing device.
As shown in FIG. 9, a shaft 401 is inserted in a bearing bore 402A of a sleeve 402 in a rotatable manner. The sleeve 402 is fixed to a base 405. The shaft 401 has a flange 403 that is formed integrally with a lower end portion of the shaft 401. The flange 403 is housed in a step portion 402D that is formed at a lower portion of the sleeve 402. The flange 403 is rotatable facing a thrust plate 404. Since the flange 403 is housed in the step portion 402D, the flange 403 works as a stopper for preventing the shaft 401 from dropping out of the sleeve 402. A lower end portion of the sleeve 402 has a lower retaining portion 402B that is processed to be like a thin cylinder, which encloses the thrust plate 404 so that the thrust plate 404 is fixed to the sleeve 402 by bending (deformed) the whole of a rim of the lower retaining portion 402B inwardly and by using adhesive.
An inner face of the bearing bore 402A is provided with dynamic pressure generating grooves 402E and 402F having a herringbone shape that is known in the technology. In addition, a face of the sleeve 402 of the flange 403 that is opposed to the step portion 402D is provided with a dynamic pressure generating groove 403A, while the face of the flange 403 that is opposed to the thrust plate 404 is provided with a dynamic pressure generating groove 403B. The gap between the shaft 401 and the flange 403 including the dynamic pressure generating grooves 402E, 402F, 403A and 403B is filled with oil 412 that is an operating fluid. There is an oil reservoir 402C between the dynamic pressure generating grooves 402E and 402F on the inner face of the bearing bore 402A. The oil 412 is stored in the oil reservoir 402C.
The shaft 401 is provided with a hub 406 for retaining a rotor magnet 407. A clamper 411 is fixed to the hub 406 by screwing an external thread 413 into an internal thread (not shown) that is provided in the shaft 401. The base 405 is provided with a stator 408 that is disposed at a position opposed to the rotor magnet 407. A disk 409, which is a medium that is used for recording and/or reproducing information, is disposed on the hub 406 via a spacer 410 and is fixed by the clamper 411.
An operation of the conventional hydrodynamic bearing device having the above-explained structure will be described below. When the stator 408 is supplied with electric power, a rotating magnetic field is generated. Thus, a torque is provided to the rotor magnet 407, so that the hub 406, the disk 409, the spacer 410, the clamper 411, the shaft 401 and the flange 403 start to rotate. As a result of this rotation, a pumping pressure is generated in the dynamic pressure generating grooves 402E, 402F, 403A and 403B, which causes floatation of the shaft 401, so that the shaft 401 can rotate without contacting the thrust plate 404 and the inner face of the bearing bore 402A.
In addition, a flangeless shaft type hydrodynamic bearing device that has a shaft without a flange has started to be adapted so as to respect recent demands for smaller motors. In this type, a radial dimension can be reduced so that all of the shaft length in the axial direction can be used for the radial bearing. Therefore, a dimension in the axial direction can also be reduced (See, for example, Japanese Unexamined Patent Publication No. 58-24616, Japanese Unexamined Patent Publication No. 59-43216, Japanese Patent No. 2509752 and Japanese Unexamined Patent Publication No. 8331796).
Concerning the conventional hydrodynamic bearing device, there is a case where the rotation of the motor generates bubbles of air that enters the fluid when the device was assembled or that was not degassed completely. If lubricating fluid like the oil 412 contains air bubbles, its ability to support a load is lowered at the bubble portion, resulting in a drop in rotation accuracy or in lubricating ability or reduction in life.
In addition, if the air bubbles are expanded in the conventional hydrodynamic bearing device due to a low pressure environment during transportation by airplane or a high temperature environment, the lubricating fluid filling the gap between the shaft and the bearing bore may be squeezed out of the hydrodynamic bearing device resulting in a leakage. For this reason, the hydrodynamic bearing device may have a potential problem of decrease of lubricating fluid and contamination of the same depositing on other components.
In addition, the above-mentioned conventional hydrodynamic bearing device needs the lower retaining portion 402B for fixing the thrust plate 404 to the sleeve 402. The lower retaining portion 402B is formed on the sleeve 402 that is manufactured by a precision process at a high cost, as the dynamic pressure generating grooves 402E and 402F are formed. Accordingly, the sleeve 402 becomes an expensive component. If a defect such as a crack occurs when the lower retaining portion 402B is processed, the sleeve 402 is wasted resulting in a large loss.
In recent years, improved productivity of the hydrodynamic bearing device has been required so as to reduce costs. Furthermore a compact and low profile which incorporates the hydrodynamic bearing device is required as demand for the disk driving apparatus, such as a hard disk drive, increases.
In addition, the hydrodynamic bearing device of the flangeless shaft type has the following potential problems. First, although it is advantageous for small size when it is adopted for a spindle motor of an inner rotor type, there is no space for attaching a stopper inside a hub. Second, since the height is also reduced along with a smaller size, the capacity for keeping oil is limited. The oil keeping space has a trade off relationship with the radial bearing dimension. Therefore, when trying to secure sufficient bearing stiffness in the device of compact size, it is difficult to secure sufficient oil keeping space and avoid a decrease in device life. Third, since a dimension in the axial direction is also reduced along with a smaller size, it is difficult to secure sufficient space for coupling the thrust plate and a thin plate. Fourth, since air bubbles may be generated in the bearing space due to the smaller size, it is necessary to reliably remove air bubbles.